20 plus years ago I did a shop refit on Chancery Lane in that London.
In the cellar, clipped across the ceiling, direct in from the road was a huge FO SWA cable, but it had no sheath...just the SWA. We had no idea what it was so we got a be-suited gibbon from the LEB down to have a look. After a look at some drawings and a bit of genital scratching he announced that " it is probably dead, just cut it off "
I passed my hacksaw to him and told him to carry on...he declined my offer!
As far as I know it is still there. We painted it orange, covered it in labels, marked it on all drawings, put a notice next to the DB with a copy of the drawing AND a couple of pics of the cable with a copy of that days newspaper next to it to give some indication of date. Never did find out what it was for
" treat it as being live because one day it will be...and you might not,"